Field of the Invention
The invention refers to a voltage controlled oscillator with a transistor and a frequency variable resonant circuit connected to the transistor.
In many RF and microwave systems receivers and transmitters are required which can be tuned over a wide frequency range. The cost and complexity of the circuitry can be considerably reduced by using a single Voltage Controlled Oscillator (VCO) or using only a few VCOs. For example, for microwave sensors employing wide band continuous phase swept frequency modulation, it is essential that a single oscillator be used. VCOs also find wide application in practically every type of RF and microwave systems. A particularly important use for wide-band VCOs is in microwave sensors which employ linear frequency sweeping. This is commonly referred to as Frequency Modulated Continuous Wave, or FMCW modulation. In order for the modulation to work, the sweep must be phase continuous and the most efficient and effective way of generating such a sweep is to apply a swept drive voltage to the tuning terminal of the VCO. The range resolution of such sensors is proportional to the bandwidth of the sweep. Therefore, in many cases, a very wide sweep bandwidth is required.
The potential tuning range is often not achieved in practice, especially when the ratio of the upper and the lower frequency is equal to or above 2:1 (so called xe2x80x9coctavexe2x80x9d tuning). This is due to the fact that the gain of the transistor drops with increasing frequency. Therefore, the circuit stops oscillating over the upper part of the resonator tuning range.
The achievable frequency range could be extended by increasing the gain of the transistor, e.g. by increasing the collector current. However, this often causes excess gain at lower frequencies leading to instability and parasitic oscillations. A common example of this is xe2x80x9csubharmonicxe2x80x9d pumping, where a large signal is generated at half the expected resonant frequency.
It is accordingly an object of the invention to provide a voltage controlled oscillator which overcomes the hereinafore-mentioned disadvantages of the heretofore-known oscillators of this general type and which permits a wide tuning bandwidth to be achieved with simple circuitry.
With the foregoing and other objects in view there is provided, in accordance with the invention, a voltage controlled oscillator, including a transistor with a controlled path with a first and a second terminal and a control terminal. A frequency variable resonant circuit has an input terminal for a tuning voltage, the resonant circuit is connected between the first terminal of the controlled path of the transistor and a terminal for a reference potential. A series circuit includes a first and a second inductive element connected between the second terminal of the controlled path of the transistor and the terminal for the reference potential. A frequency dependent feedback path coupled to a circuit node between the first and the second inductive elements and the control terminal of the transistor. The feedback path includes a series circuit of a first and a second capacitive element and a resistor connected to the circuit node between the first and the second inductive elements and to the terminal for the reference potential. A third inductive element connects the circuit node between the capacitive elements to the control terminal of the transistor.
In one embodiment of the voltage controlled oscillator has an output terminal connected via a blocking capacitor to the second terminal of the transistor.
In another embodiment of the voltage controlled oscillator the first capacitive element of the feedback path is connected to the first circuit node and the second capacitive element is connected, via the resistor, to the reference potential terminal.
In another embodiment the resonant circuit of the voltage controlled oscillator includes a first and a second variable capacitance connected in series and a further inductive element connected in parallel to the first variable capacitance and the second variable capacitance. The input terminal of the resonant circuit is connected to a circuit node located between the first variable capacitance and the second variable capacitance.
In another embodiment the variable capacitances of the voltage controlled oscillator are varactor diodes. The respective cathode of the varactor diodes are connected together at a circuit node located between the first variable capacitance and the second variable capacitance.
In another embodiment the voltage controlled oscillator has a bipolar transistor. The first terminal of the transistor is an emitter connected to the resonant circuit and the second terminal of the transistor is a collector connected to the first inductive element.
In another embodiment the voltage controlled oscillator has a field effect transistor with a drain-source-path connected to the resonant circuit and to the first inductive element.
This objective is solved by a Voltage Controlled Oscillator, including a transistor with a controlled path having a first and a second terminal and a control terminal; a frequency variable resonant circuit with an input terminal for a tuning voltage, the resonant circuit being connected between the first terminal of the controlled path of the transistor and a terminal for a reference potential; a series circuit including a first and a second inductive element connected between the second terminal of the controlled path of the transistor and the terminal for the reference potential; a frequency dependent feedback path coupled to a circuit node between the first and the second inductive elements and the control terminal of the transistor; the feedback path including a series circuit of a first and a second capacitive element and a resistor connected to the circuit node between the first and the second inductive elements and to the terminal for the reference potential and having a third inductive element connected to a circuit node between the capacitive elements and to the control terminal of the transistor.
The feedback path of the oscillator includes a voltage divider with respect to the AC signal. This forms a potentiometer chain supplying the output signal of the VCO to the transistor base. The amount of feedback varies with the frequency. The feedback increases with frequency so that the gain increases at higher frequencies. As an advantage, subharmonic pumping is avoided. The amount of feedback can be adjusted according to a suitable choice of the values for the elements in the feedback path. Overall this leads to a wide tuning range for the oscillator.
The feedback path includes a first and a second capacitor and a resistor connected in serial fashion. The circuit node between the capacitors is connected to the control input of the transistor of the oscillator via an inductive element. The transistor can be of either type, e.g. a NPN bipolar transistor, a PNP bipolar transistor, or a field effect transistor.
The resonant element connected to the emitter of the bipolar transistor or the source of the field effect transistor has a tuning terminal for the reception of a control voltage. The voltage varies the characteristics of the resonant circuit. The resonant circuit includes a series connection of two varactor diodes with an inductive element connected in parallel. The tuning voltage is applied to the connection node between the varactors.
The invention provides a Voltage Controlled Oscillator which can be tuned over a wide frequency range without subharmonic pumping. Only one Voltage Controlled Oscillator is required to cover the tuning range for microwave sensors which employ linear frequency sweeping. The circuit employs relatively few elements and is relatively simple.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage controlled oscillator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.